Thermal switch



De. 3, 1963 H. J. J. VAN BOORT ETAL 3,113,194

THERMAL SWITCH Filed Aug. 3, 1959 FIG. 2

FIG. 1

Hznrlcus Johannes Joseph Van Boon Johannes Hcndrikus Hildcrink INVENTOR United States Patent The present invention relates to thermal switches. More particularly, the invention relates to a thermal switch comprising two connecting terminals which, in the cold state of the switch, are connected through a heating member, the resistance of which at a higher applied voltage is lower than in the case of a lower applied voltage, and which switch comprises a heat sensitive element which, when heated, is adapted to close a contact which is open when cold, thus establishing between the terminals a practically resistanceless connection which is interrupted after some predetermined time.

When using such switches as a starter switch for igniting electric gas and/ or vapour discharge tubes, the switch is generally connected in series combination with an impedance which may entirely or partially consist of an inductance. While the switch is connected in parallel with the tube and in series with the impedance and, if desired, with at least one thermionic electrode of the tube, it is found that under unfavourable conditions, for example, a low ambient temperature and/ or a low supply voltage, the tubes may fail to strike when using a thermal switch comprising a heating member, the resistance of which depends upon the voltage applied to it.

The present invention has for one of its objects to provide an improved thermal switch which mitigates and/or obviates the above-described disadvantages.

The invention features, inter alia, a second contact which is closed in the cold state and is then connected in series with the heating member, while the contacts are controlled and arranged so that the heating member does not constitute an electrical connection between the terminals during interruption of the practically resistanceless connection.

The invention is based on the recognition that the ignition difficulties are to be attributed to the voltage-dependent resistance of the heating member. On interruption of the substantially resistanceless connection, the inductance delivers a voltage pulse which has to discharge through the tube but is impeded in doing so by the heating member. As a matter of fact, this member has a low resistance at a high voltage and in conventional devices this resistance lies in parallel with the tube. Thus, the heating member consumes the major part of the voltage intended for igniting the tube and the voltage set up across the tube is insufiicient to ignite the tube. According to the invention, provision is made that the heating member is inelfective during interruption of said connection.

In a suitable embodiment of the invention, the substantially resistanceless connection comprises the series combination of the two closed contacts and this connection is broken by the second contact at an instant at which the heating member is short-circuited by the first contact. In this case the heating member is prevented from being connected to the terminals by the opening second contact. The short-circuit serves to interrupt a heavy current and so to produce a high voltage.

In order to prevent the ignited tube from becoming 3 ,113,194 Patented Dec. 3, I963 the closed first contact which is closed upon interruption of the series combination of the heating member and the second contact, this series combination being reestablished only after breaking the first contact. In this case the heating member is prevented from establishing a connection between the terminals by the open second contact.

According to the invention, excellent results are obtained if the second contact is made and broken abruptly. This can be achieved by means of a spring or magnet structure and results in the switch reliably repeating the normal switch cycle if the tube fails to strike at once.

In this case, the heat-sensitive element is preferably made up of two bi-metallic strips which extend in line with each other, overlap each other and move in opposed directions upon being heated, one of these strips being in heat-transferring relationship with the heating member, while the heat of the heating member is transferred to the other strip through the first strip. The use of this bi-metal structure permits in a simple manner adjustment of the time period for which the tube is allowed to strike and/ or the initial state of the switch to be reestablished. Should this time period be limited by the cooling of the heating member, it may become unduly long due to the heat capacity of this member.

Furthermore, the invention concerns a device comprising an electric gas and/ or vapour discharge tube serially connected to an impedance which may entirely or partially consist of an inductance, the device further comprising an automatic ignition switch which is connected in parallel with the discharge tube and in series with the impedance and, if desired, with at least one thermionic electrode of the tube. According to the invention, the ignition switch is a thermal switch as set out hereinbefore.

In order that the invention may be readily carried into effect, examples will now be described in detail with reference to the accompanying drawing, in which:

FIG. 1 is a side view of an embodiment of the thermal switch of this invention in which the heating member is short 'circuited when the substantially resistanceless connection across the terminals is interrupted;

FIG. 2 is a partial front view of the thermal switch of FIG. 1;

FIG. 3 is a side view of another embodiment of this invention, in which the heating member is connected in a unipolar manner; and

FIG. 4 is a partial front view of the embodiment of FIG. 3.

Terminals 2, 3 of the thermal switch shown in FIG. 1 are secured to an insulating base plate 1. Terminals 2, 3 are connected to thermionic electrodes 4 of a gasdischarge tube 5, for example, a fluorescent lamp of 40 W. The thermal switch is connected in parallel with tube 5 and, moreover, in series with the thermionic electrodes'4, a choke coil 6 and a manually operated switch 7 to a suitable supply 8, 9 for example, voltage mains of 50 or 60 c./s. and 220 v.

As is known, the thermionic electrodes after connection to the supply, and with a substantially resistanceless connection between the terminals 2, 3 of the switch, are traversed by a heavy current, which is determined by the impedance of the choke coil 6 and the (usually negligible) impedance of the thermionic electrodes 4, and are heated to emission temperature by this current. Upon interruption of this heavy current a rather strong voltage pulse may be produced by the choke coil, which pulse causes 7 the tube to ignite.

extinguished, the short-circuit of the heating member is substantially resistanceless connection extends only over I It will be evident that the substantially resistanceless connection may have a little resistance without appreciably reducing said [heavy current provided that this resistance be low and does not, for example, exceed 10% to 20% of the impedance of the choke coil.

The switch shown in the drawing comprises two contact members 10, 11 which are in contact with each other in the cold state of the switch, a heating member 12 and a thermo-responsive element made up of two bimetallic strips 13, 14. The contact member is supported by the suitably formed metal strip 15 and electrically is alsoconnected to the terminal 2 via the same metal strip 15. The contact member 11 is supported by the shoulder portion 16 of an M-shaped metal spring (see FIG. 2) and is electrically connected to the heating member 121 through this spring and a support 17. The latter comprises two V-shaped incisions 18 in which the lower ends of the outer limbs 19, 20 of the M-shaped spring are engaged while the middle limb 21 is secured to a vertical face of the support 17. This support 17 furthermore has a bent-over edge 22 for supporting the heating member 12. If desired, a resilient and electrically conductive intermediate member may be provided between the support 17 and the heating member 12. The heating member 12 is connected by way or the bimetallic strip 13 to the terminal 3 through an L-shaped metal strip 2 3, the horizontal part of which extends between limbs 24 of the support 17, which limbs are secured to the base plate 1, the strip 23 being electrically insulated from the limbs 24 (see FIG. 2).

The bimetallic strips 13 and 14 extend in line with each other and overlap each other so that, on heating, the strip 13 moves to the right and the strip 14 moves to the left, as shown by the arrows in the drawing. The upper free end of the strip 14 comprises a contact member 25 adapted to make contact with the contact member 11.

The heating member 12. is disc-shaped and consists of a material, for example, such as a mixture of silicon carbide and graphite, the resistance of which, if a higher voltage is applied to it, is lower than when a lower voltage is applied to it. The resistance value is such that the heating member, when applying a voltage to the device, is capable of taking a power, for example, 15 w, sufficient for rapidly establishing the substantially resistanceless connection.

During normal operation of the tube, when as is known, approximately only half the supply voltage is set up across it, a voltage-independent resistance would take up approximately one-fourth of the first-mentioned power. This means an undue loss and moreover entails the risk of the switch short-circuiting the ignited tube. However, a voltage-dependent resistance permits the power taken by a tube during operation to be reduced to a few percent of the first-mentioned power.

In practice, a resistance of approximately 300082- at 220 v., and approximately 25,0000- at 110 v. has proved appropriate. On the occurrence of the voltage pulse produced by the choke coil 6 and amounting, for example, to more than 1000 v., the voltage-dependent resistance of the heating member 12 is low so that ignition of the tube may be impeded.

In accordance with the invention, the voltage pulse is prevented from occurring across the voltage-dependent heating member.

The device shown in FIGS. 1 and 2 operates as follows:

Upon closing the manually operated switch 7, the bimetallic strip 13 is heated by the heating element 12 and deflected to the right so that the contact member 25 at the upper end of the still cold bimetallic strip 14 contacts with the contact member 11 on the shoulder 16 of the M-shaped spring (see FIG. 2). The heating member 12 is then short-circuited and the substantially resistanceless connection is established between the terminals 2 and 3 of the thermal switch. This connection extends over the closed contact pairs 25, 11 and 11, 10.

The current through the thermionic electrodes 4jwhich current was initially limited by the resistance of the heating member and was approximately 0.075 A. in the given example, is now for all practical purposes limited only by the choke coil and amounts to approximately 0.65 A. in the given example, so that the thermionic electrodes can be heated to their emission temperature within 1 to 2 seconds. The contact member 25 still exerting a pressure to the right overcomes the contact pressure between the contact members 10 and 11 after said 1 to 2 seconds causing spring 16 to jump against the left-hand side of a member 26. Member 26 is connected to the contact member 25 and, for example, together with the contact member 25, may form a U-shaped bracket. Interruption of the contact 10, 11 is followed by interruption of the contact 25, 11.

Upon interruption of the contact between the contact members 10* and 11 there is no electrical connection between the terminals 2 and 3 so that the voltage pulse produced upon interruption may fully serve for igniting the tube 5.

After production of the voltage pulse, the switch should resume its initial position as rapidly as possible in order that a non-ignited tube may again be caused to ignite. This is effected by means of the bimetallic strip 14 to which sufiicient heat has meanwhile been transferred through the bimetallic strip 13 for causing the contact member 25 to move to the left. The better the heat transfer between the bimetallic strips 13 and 14 takes place, the sooner will the contact member 25 return. The member 26 also returns, taking along the contact member 11 and causing contact member 11 to return to the contact member 10. Since the contact 25, 11 is already open, the initial condition is reestablished. It is important for the closure of the contact 10, -11 to be preceded by removing the short-circuit from the heating member 12, lest the operative tube 5 be extinguished.

The spring system 19, 20, 21 is a conventional system with the highly unstable neutral position between the two extreme positions of the contact member 1 1, between which this member changes its position abruptly. For returning to the contact member 10, the member 26 has to exert a force directed to the left on the system.

The terminals 2 and 3 may be connected through a radio interference suppression capacitor 27. In order that the voltage pulse not be reduced excessively by this capacitor, it should have a high impedance. A capacitance value of approximately 10,000 micro-microfarads proved to be permissible.

The switch shown in FIGS. 3 and 4 comprises a contact 30, 31 closed in the cold state, a contact 32, 33 open in the cold state, a disc-shaped heating member 34 having a voltage-dependent resistance, a heat responsive element made up of bi-metallic strips 3536 which extend in line with each other, overlap each other and move in opposed directions on heating (as indicated by the direction of the arrows in FIG. 3) and a switch arm consisting of an M-shaped metal spring 37, 38, 39.

The movable contact members 31, 32 are secured to the shoulder portion 40 of the switch arm through which they are made to contact with either of the two stationary contact members 30 and 33.

The terminals 2, 3 are connected together, with the interposition of the heating member 34, through the closed contacts 30, 31. However, the contacts 32, 33 when closed connect the terminals substantially without intermediate resistance. The electrical connection between the right-hand contact face (FIG. 3) of the heating member 34 and the terminal 2 extends right through the bi-metallic strip 35 and the contact member 30 connected thereto, the closed contact 30, 31, the contact arm 37, 38, 39 and a metal support 41, 42, 43. The lefthand contact face of the heating member is connected to the terminal 3 through a metal clamp 44, 45 and an adjoining metal strip 46. The contact member 33 is constituted by part of the strip 46.

I The lower end of the bi-metallic strip 36 rests on an insulating strip 47 on the middle limb 37 of the switch arm. Provision is made of a C-shaped frame 48 of insulating material which is carried by the support 41, 42, 43 and the strip 46. The upper parts 41, 42 of the support are furnished with \l-shaped incisions in which the side limbs 38, 39 of the switch arm are engaged.

When a voltage is applied to the switch, the contact 30, 31 is closed. The serially connected heating member 34 heats the bi-metallic strip 35 which, through the still cold bi-metallic strip 36, urges the contact member 32 on the switch arm into contact with the contact member 33 of strip 46. The heat from the heating member is transferred through the bi-metallic strip 35 to the bimetallic strip 36 and causes the latter to curve in opposite direction, thus reducing the pressure exerted on the resilient switch arm so that the latter is allowed to return abruptly. This movement results in first breaking the contact 32, 33 and in subsequently closing the contact 30, 31 thus re-establishing the initial condition. Also in this case, as a consequence, the heating member 34 does not establish an electrically conductive connection between the terminals 2, 3 during interruption of the substantially resistanceless connection.

The M-shaped switch arm is a conventional spring system which, when exerting sufiicient pressure on the middle limb 37, jumps to the stationary contact 33, and, upon a given decrease of this pressure, jumps back to the stationary contact member 30. This requires the neutral position of the spring system to be located beyond the stationary contact members 36 and 33, in particular on the side of the contact member 33.

While we have described the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. A thermal switch comprising first and second input terminal means, a first contact member comprising a contact portion, a second contact member comprising a contact portion and a snap switch element, a resistor heating element having electrical resistance variations as determined by variations of the voltage applied thereto, means for connecting said heating element and said first and second contact members in series circuit relationship to said input terminal means, and a thermally responsive element coupled in heat conducting relationship with said heating element and selectively connecting and disconnecting said voltage responsive heater element from said series circuit, said thermally responsive element and at least one of said contact members coacting to establish a substantially resistanceless connection between said input terminal means after said heater element is disconnected from said series circuit.

2. A thermal switch comprising first and second input terminal means, a first contact member comprising a contact portion, a second contact member comprising a contact portion and a snap switch element, a resistor heating element having electrical resistance variations as determined by variations of the voltage applied thereto, means for connecting said heating element and said first and second contact members in series circuit relationship to said input terminal means, and a thermally responsive element coupled in heat conducting relationship with said heating element for selectively shortcircuiting said voltage responsive heater element from said series circuit.

3. A thermal switch comprising first and second input terminal means, a first contact member comprising a contact portion, a second contact member comprising a contact portion and a snap switch element, a third contact member comprising a contact portion, a resistor heating element having a high electrical resistance value upon applying a relatively low voltage thereto and a low electrical resistance value upon ap plying a high voltage thereto, means for connecting saidheating element and said first'and second contact members in series circuit relationship to said input terminal means, and a thermally responsive element coupled in heat conducting relationship with said heating element for selectively connecting and disconnecting said voltage responsive heater element from said series circuit said thermallyresponsive element and at least one of said contact members selectively coacting to establish a substantially resistanceless connection between said input terminal means.

4. 'A thermal switch comprising first and second input terminal means, a first contact member comprising a contact portion, a second contact member comprising a contact portion and a snap switch element, a heating element having a high electrical resistance value upon applying a relatively low voltage thereto and a low electrical resistance value upon applying a high voltage thereto, means for connecting said heating element and said first and second contact members in series circuit relationship to said input terminal means, and a thermally responsive element comprising two bi-metallic strips arranged in line, the first of said bimetallic elements being movable towards the said contact portion of said second contact member and being arranged in heat conducting relationship with said heating element, and the second of said bi-metallic elements having a contact portion engaging the contact portion of said contact member thereby to selectively short-circuit said heating element and being movable away from the said contact portion of said second contact member.

5. A thermal switch comprising first and second input terminal means, a first contact member comprising a contact portion, a second contact member comprising a contact portion and a snap switch element, a third contact member comprising a contact portion, a resistor heating element having electrical resistance variations as determined by the level of voltage applied thereto, means for connecting said heating element and said first and second contact members in series circuit relationship to said input terminal means, a thermally responsive element coupled in heat conducting relationship with said heating element and responsive thereto, said thermally responsive element being coupled to said third contact member for selectively closing said second and third contact members for a predetermined time and selectively short circuiting said heater element from said series circuit thereby establishing a substantially resistanceless connection between said input terminal means, said thermally responsive element coacting with said snap switch element to rapidly interrupt said substantially resistanceless connection and establish an open circuit condition across said input terminals for a second predetermined length of time, and means operative after said'second predetermined length of time and including said thermally responsive element for restoring to said input terminal means said series circuit comprising said first and second contact members and said heating element.

6. A thermal switch comprising first and second input terminal means, a first contact member comprising a contact portion, a second contact member comprising a contact portion and a snap switch element, a resistor heating element having electrical resistance variations as determined by the level of voltage applied thereto, means for connecting said heating element and said first and second contact members in series circuit relationship to said input terminal means, a thermally responsive element coupled in heat conducting relationship with said heating element and responsive thereto for selectively short circuiting said heater element from said series circuit and for simultaneously establishing a substantially resistanceless connection between said input terminal means, said thermally responsive element coacting with said snap switch element at a timewhen said heating element is short circuited to rapidly open said first and second contact members thereby interrupting said substantially resistanceless connection and establishing an open circuit condition across said input terminal means.

References Cited in the file of this patent Zierahn Mar. 8, 1938 8 Eaton Aug. 22, 1939 Gustin et a1. Feb. 18, 1941 Hand July 8, 1941 Stimson Nov. 29, 1942 Cook Feb. 29, 1944 Townsend Mar. 20, 1945 Thomson Nov. 29, 1949 Bocciarelli June 18, 1957 

5. A THERMAL SWITCH COMPRISING FIRST AND SECOND INPUT TERMINAL MEANS, A FIRST CONTACT MEMBER COMPRISING A CONTACT PORTION, A SECOND CONTACT MEMBER COMPRISING A CONTACT PORTION AND A SNAP SWITCH ELEMENT, A THIRD CONTACT MEMBER COMPRISING A CONTACT PORTION, A RESISTOR HEATING ELEMENT HAVING ELECTRICAL RESISTANCE VARIATIONS AS DETERMINED BY THE LEVEL OF VOLTAGE APPLIED THERETO, MEANS FOR CONNECTING SAID HEATING ELEMENT AND SAID FIRST AND SECOND CONTACT MEMBERS IN SERIES CIRCUIT RELATIONSHIP TO SAID INPUT TERMINAL MEANS, A THERMALLY RESPONSIVE ELEMENT COUPLED IN HEAT CONDUCTING RELATIONSHIP WITH SAID HEATING ELEMENT AND RESPONSIVE THERETO, SAID THERMALLY RESPONSIVE ELEMENT BEING COUPLED TO SAID THIRD CONTACT MEMBER FOR SELECTIVELY CLOSING SAID SECOND AND THIRD CONTACT MEMBERS FOR A PREDETERMINED TIME AND SELECTIVELY SHORT CIRCUITING SAID HEATER ELEMENT FROM SAID SERIES CIRCUIT THEREBY ESTABLISHING A SUBSTANTIALLY RESISTANCELESS CONNECTION BETWEEN SAID INPUT TERMINAL MEANS, SAID THERMALLY RESPONSIVE ELEMENT COACTING WITH SAID SNAP SWITCH ELEMENT TO RAPIDLY INTERRUPT SAID SUBSTANTIALLY RESISTANCELESS CONNECTION AND ESTABLISH AN OPEN CIRCUIT CONDITION ACROSS SAID INPUT TERMINALS FOR A SECOND PREDETERMINED LENGTH OF TIME, AND MEANS OPERATIVE AFTER SAID SECOND PREDETERMINED LENGTH OF TIME AND INCLUDING SAID THERMALLY RESPONSIVE ELEMENT FOR RESTORING TO SAID INPUT TERMINAL MEANS SAID SERIES CIRCUIT COMPRISING SAID FIRST AND SECOND CONTACT MEMBERS AND SAID HEATING ELEMENT. 